Method and Apparatus for Passive Detection of Near-Surface Human-Scale Underground Anomalies Using Earth Field Measurements

ABSTRACT

A method for detecting a subsurface anomaly at a near-surface depth, comprises positioning an electromagnetic sensor configured to measure a component of a planetary electromagnetic field such that the electromagnetic sensor is suspended just above a ground-air barrier and does not contact a ground surface; selecting an electromagnetic frequency by calculating a function of properties of the ground that include relative permittivity, relative permeability, and resistivity; moving the electromagnetic sensor over the surface of the ground; repeatedly measuring intensity of the component of the planetary electromagnetic field at the frequency to obtain a set of measurements; and comparing at least a first measurement in the set of measurements to at least a second measurement in the set of measurements to identify a change in the intensity of the component of the planetary electromagnetic field that is indicative of a presence of a subsurface anomaly.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application61/790,937, entitled Detection of Underground Structures Using EarthField Measurements, filed on Mar. 15, 2013, and incorporated byreference as if fully rewritten herein.

FIELD

This application relates to passive detection of human-scale undergroundstructures at near-surface depths using earth field measurements.

BACKGROUND

It is often desirable to sense the location of subsurface objects fromoutside of the surface of the material in which it is encased (i.e. anobject buried underground). For example, sensing the presence ofhuman-scale subsurface objects (both metallic and non-metallic) atrelatively near-surface depths (i.e., between zero and 30 meters) cansave time, costly explorative excavation, and avoid possible damage tosubsurface objects through unguided excavation. Dangers related todigging up objects, such as explosive land mines or gas utility lines,do not have to be contended with or can be reduced if remote sensingfrom the surface locates the object prior to excavation.

A number of methods have been developed to locate subsurface objects.Subsurface objects, which can be referred to as anomalies, may havevarious compositions and also include an air pocket or any void orvolume uniquely different than the surrounding homogeneous orpredictably non-homogeneous material. Metallic objects can be foundrelatively easily with devices such as metal detectors and through ahost of other technologies, such as Ground Penetrating Radar (GPR). Itis, however, much more challenging to find non-metallic subsurfaceobjects. The invention described herein is a passive method andapparatus for detecting both metallic and non-metallic subsurfaceobjects, voids and other anomalies using the natural electromagneticsignal emanating from Earth's interior.

The Earth's interior is a highly dynamic structure comprised of multiplelayers with a fluid behavior. As the Earth rotates, portions of thisfluid move at different velocities and directions. This motion (as wellas other factors including lightning, solar wind and flares, etc.)generate low level electromagnetic signals, which then travel outwardand pass through the Earth's surface. One example of this phenomena isthe well-known core-dynamo effect that creates the quasi-steady stategeomagnetic field within the planet. Heating, conduction, and swirlingof molten rock can also produce mechanical and electrical signals thattravel towards the surface. As these signals travel towards the Earth'ssurface, they will be affected by the material through which theytravel. This effect may show up as variations in signal strength, signalphase, frequency, etc. As the composition of the material varies, sowill its effects on the signal passing through it. By monitoring, overan area, the signals emanating from below the Earth's surface, materialvariations can be detected. This effect can be employed and adapted tolocating subsurface objects, voids or other anomalies.

One method of detecting underground structures and other anomalies isaudio magneto tellurics (AMT), which monitors AC-signals in the audiofrequency range to discover extremely large-scale geological structures.These structures, referred to herein as being of geologic scale,include, by way of example, layers of mineral deposits, rock formations,or other natural resources (such as, for example, coal seams). AMT andother known techniques may not be effective for detecting subsurfaceobjects on smaller scales, at higher resolutions, or at shallowerdepths.

Another method for detecting underground structures and other anomaliesis passive magneto tellurics, which relies on natural, lightening-drivenatmospheric noise signals, such as lightening and magnetosphereactivities. U.S. Pat. Nos. 4,507,611, 4,825,165 and 5,148,110 to Helms,et al., which are incorporated herein by reference in their entireties,disclose such and other methods for detecting subsurface anomalies. U.S.Pat. No. 6,414,492 to Myers, describes another method for detectinggeophysical discontinuities in the Earth by measuring the electricalcomponent of the Earth's electromagnetic field at frequencies below 5kHz.

These identified methods are capable, to varying degrees, of detectinglarge, or geologic-scale anomalies at significant sub-surface depths.For example, passive magneto tellurics can detect geological-scaleanomalies starting at depths from a few tens of meters to manykilometers, but lacks the resolution to detect small, human-scaleobjects. Similarly, the passive method disclosed in U.S. Patent No.5,414,492 can detect geologic-scale anomalies at depths greater than22.5 meters. The identified methods are not, however, capable ofdetecting human-scale anomalies or detecting both metallic andnon-metallic anomalies at more shallow, near-surface depths (i.e.,between zero and 30 meters). For example, none of these methods issufficiently capable of detecting human-scale anomalies, such as plasticpipes, storage tanks, land mines, or other man-made objects (referred toherein as human-scale objects), buried at near-surface depths. Moreover,the identified methods are capable of generating only relativelylow-resolution representations or images of detected subsurfaceanomalies and have limited capability for determining characteristics ofdetected subsurface anomalies, such as composition.

Thus, there exists a need in the art for methods and apparatus topassively detect human-scale anomalies, to detect both metallic andnon-metallic anomalies, to detect anomalies at near-surface depths, toprovide higher resolution representations or images of detectedsubsurface anomalies, and to determine characteristics of detectedsubsurface anomalies, such as composition, than what presently is knownor available in the art.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key or essentialfeatures of the claimed subject matter, nor is it intended to limit thescope of the claimed subject matter.

In one embodiment, a method for detecting a human-scale, subsurfaceanomaly at near-surface depths below an area of the Earth's surfacecomprises suspending a sensor for measuring a component of the Earth'selectromagnetic field proximate to the ground-air barrier, measuring theintensity of a component of said field over the area at a frequency of 5kHz or greater, and comparing the measurements to identify variations inthe intensity of the field within the area to detect the presence of ahuman-scale, subsurface anomaly at a near-surface depth below the firstarea.

In one embodiment, the presence of an anomaly is detected by comparingmeasurements of the electric component of the Earth's electromagneticfield at a frequency of 5 kHz or greater. In another embodiment, thepresence of an anomaly is detected by comparing measurements of themagnetic component of the Earth's electromagnetic field at a frequencyof 5 kHz or greater. The presence of an anomaly also may be detected bycomparing measurements of the electric and magnetic components of theEarth's electromagnetic field at a frequency of 5 kHz or greater. In oneembodiment, the method may include using variations in anelectromagnetic property to determine characteristics of the detectedanomaly. Variations in an electromagnetic property may be detected usingan array of sensors that may comprise a plurality of sensors, and thelocation of each measurement may be determined, at least in part, usingtriangulation.

In one embodiment, a method for determining the depth of a human-scale,subsurface anomaly at near-surface depths below a first area of theEarth's surface comprises suspending a sensor for measuring a componentof the Earth's electromagnetic field proximate to the ground-air barrierwithin the area, measuring the intensity of a component of said fieldover the area at a plurality of frequencies of 5 kHz or greater,identifying the frequency demonstrating the greatest change in intensityin the presences of the anomaly, calculating the depth of the anomalyusing the identified frequency, and determining one or morecharacteristic of the composition or makeup of said anomaly with a mostlikely material based on a host of possible materials.

In one embodiment, the depth of an anomaly is determined by measuringthe intensity of the electric component of the Earth's electromagneticfield at a plurality of frequencies of 5 kHz or greater. In anotherembodiment, the depth of an anomaly is determined by measuring theintensity of the magnetic component of the Earth's electromagnetic fieldat a plurality of frequencies of 5 kHz or greater. In yet anotherembodiment, the depth of an anomaly is determined by measuring theintensity of the electric and magnetic components of the Earth'selectromagnetic field at a plurality of frequencies of 5 kHz or greater.In one embodiment, the method may include using variations in anelectromagnetic property to determine characteristics, such as size,shape and material composition, of the detected anomaly. Variations inan electromagnetic property may be detected using an array of sensorsthat may comprise a plurality of sensors, and the location of eachmeasurement may be determined, at least in part, using triangulation.

Another embodiment is an apparatus for detecting human-scale objectsbelow the surface of the Earth at near-surface depths that comprises asensor for measuring a component of the Earth's electromagnetic field atfrequencies greater than 5 kHz, a frequency-selective circuit, anamplifier, and a recording device. In one embodiment, the sensor iscapable of measuring the electric component of the Earth'selectromagnetic field at a frequency of 5 kHz or greater, and in anotherembodiment, the sensor is capable of measuring the magnetic component ofthe Earth's electromagnetic field at a frequency of 5 kHz or greater. Inan alternate embodiment, the sensor is capable of measuring the electricand magnetic components of the Earth's electromagnetic field at afrequency of 5 kHz or greater. One embodiment further includes areceiver for determining the location of each measurement usingtriangulation, or other means for determining position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the typical electric and magnetic field radiatingfrom the Earth.

FIG. 2 illustrate the difference in intensity versus position fortypical electromagnetic fields radiating from the Earth.

FIG. 3 illustrates an example of distortions of the Earth'selectromagnetic field.

FIG. 4 illustrates a second example of distortions of the Earth'selectromagnetic field.

FIG. 5 illustrates how an object located below the air-ground barriercan distort components of the Earth's electromagnetic field.

FIG. 6 illustrates the effects of an object located below the air-groundbarrier on the electric component of the Earth's electromagnetic fieldat varying frequencies.

FIG. 7 illustrates the typical electric and magnetic fields radiatingfrom the Earth in the presence of a non-magnetic anomaly.

FIG. 8 illustrates the differences in intensity versus position for theelectric and magnetic components of the typical electromagnetic fieldradiating from the Earth in the presence of a non-magnetic anomaly.

FIG. 9 illustrates the typical electric and magnetic fields radiatingfrom the Earth in the presence of a magnetic anomaly.

FIG. 10 illustrates the differences in intensity versus position for theelectric and magnetic components of the typical electromagnetic fieldradiating from the Earth in the presence of a magnetic anomaly.

FIG. 11 illustrates distortions in the electric field caused by anobject with a strong dielectric constant.

FIG. 12 illustrates an exemplary sensor configuration.

FIG. 13 illustrates an exemplary circuit for detecting the presence of ahuman-scale anomaly at a near-surface depth.

FIG. 14 illustrates a field mill sensor for measuring components of theEarth's electromagnetic field.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingfigures. Other embodiments may be utilized and structural and functionalchanges may be made without departing from the respective scope of theinvention. Moreover, features of the various embodiments may be combinedor altered without departing from the scope of the invention. As such,the following description is presented by way of illustration only andshould not limit in any way the various alternatives and modificationsthat may be made to the illustrated embodiments and still be within thespirit and scope of the invention.

Because of its properties of an electromagnetic resonator, the Earth hastime-varying electric and magnetic fields. As shown in FIG. 1, theEarth's electromagnetic field 100 includes electric field component 110and magnetic field component 120, both of which has three components,one in the x-, the y-, and the z-directions. Typically, all threecomponents will travel through the Earth, and penetrate the ground-airbarrier 130. However, because electric fields tend to radiate radiallyfrom the Earth (as with every spherical object), the z-component ofelectric field 110 will ordinarily be the strongest by orders ofmagnitude.

On a clear day, the Earth's electric field has an approximate strengthof 100 V/m, and its magnetic field has an approximate strength of 0.25to 0.65 gauss. Thus, given a network of sensors located just about thesurface of the Earth, the response of the electric and magnetic fieldsshould be essentially constant over a given area of the Earth's surface,as illustrated in FIG. 2 for electrical field E and magnetic field M(both at all frequencies within the range). These intensities may varybased on the frequency being measured, with lower (fundamental)frequencies typically having higher intensities and lower (harmonic)frequencies having lower intensities. The existence of subsurfaceobjects and other anomalies will cause distortions in theseelectromagnetic fields due to differences in the electromagneticproperties of the subsurface object and its surroundings. Accordingly,both the electric and the magnetic components of the Earth'selectromagnetic field can convey information that is useful forpassively detecting relatively smaller scale (i.e., human-scale)subsurface objects and anomalies at relatively near-surface depths(i.e., less than 30 meters).

Electromagnetic waves abide by the same properties as other waves innature. These include superposition and elimination, attenuation, aswell as a host of others. Some properties, such as attenuation whentraveling through mediums, are frequency based. The concept is known asskin effect, and can be described by Equation 1, as follows:

$\begin{matrix}{\delta = {\sqrt{\frac{2\; \rho}{\omega \; \mu}}\sqrt{\sqrt{1 + \left( {\rho \; \omega \; \varepsilon} \right)^{2}} + {\rho \; \omega \; ɛ}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

-   -   where δ is the skin depth, ρ is the resistivity, ω is the        angular frequency (2π*frequency of operation), ε is the total        permittivity, and μ is the total permeability of the material.        By measuring electromagnetic signals emanating from the Earth at        closely spaced locations just above the ground, it is possible        to determine if and where a subsurface object, void, or other        anomaly exists. The signal modification, be it by attenuation in        magnitude or some other electromagnetic property, is most        pronounced in close proximity to and directly above a subsurface        object. This is a result of the general vertical direction in        which the signal propagates and the refractive nature of the        ground-air boundary at relatively low frequencies.

Two examples of distortions of the electromagnetic field are illustratedin FIGS. 3 and 4. The source of the electromagnetic fields beingdistorted can be varied in origin. The electromagnetic environment maybe composed of any number of varying field components, including but notlimited to natural phenomena such as lighting strikes, the quasi-staticearth-to-ionosphere potential and many other internal effects orman-made signals, such as those that are emanating from electric powerlines, or electromagnetic fields deliberately created in the vicinity ofthe object with a signal generator, that could be used for sensingpurposes. The distortions in these fields will be functions of frequencyand location. The nature of the distortions will be foremostcharacterized by the differences in material properties between thesubsurface object and its surroundings. The most influential of theseproperties include conductivity, dielectric permittivity, and magneticpermeability. These properties are, in turn, potentially affected byother properties, such as material porosity and moisture content. Anapparatus with at least one, but preferably many, field probe(s) cansense such distortions in said fields. Suitable electromagnetic fieldprobes can include transducers, such as electric field probes ormagnetic pickup coils, among many other options. The signals sensed overan area may be recorded and processed with a computational device toestimate the location and nature of the subsurface object. In order forsaid field distortions to be measurable, the sensing probe should be inclose proximity to the surface, and the object may not be located toodeeply or too distant from the probes, and abide by good practice incollecting and recording electronic signals. Note that frequencydependent conductivity of the surrounding material may act as asufficient shield and suppress distortions at certain frequencies.

FIG. 5 illustrates this general idea. Object 500 is located relativelynear the Earth's surface, denoted by ground-air boundary 510 and in thefield of composite signal 520 emanating from within the Earth. Thepresence of object 500 results in attenuation of composite signal 520that is detectable above the surface of the Earth in area 530. Possiblerefraction of signal 520 about object 500 may create more complicatedeffects than the simple “shadowing” illustrated. A suitable sensor canbe employed as described herein to detect the alteration in compositesignal 520 resulting from the presence of object 500. The alterationswill contrast measurements in regions where composite signal 520 has notbeen affected by the presence of object 500.

Based on the effective skin depth, or the depth of an anomaly, both theelectric and magnetic field strength intensities can be detected bystimulating a range of frequencies. As illustrated in FIG. 6 withrespect to the Earth's electric field, object 600 is located below theEarth's surface and within its electric field, shown as flux field 610.Measuring the intensity of electric field 610 for a range of frequenciesf₁, f₂ and f₃ over a given distance or area, as shown in FIG. 6, theexistence of object 600 causes a change in intensity for each offrequencies f₁, f₂ and f₃. The detectable changes in intensity areindicative of the presence of object 600. Moreover, because the changeis greatest for frequency f₂, the depth of object 600 can be determinedaccording to Equation 1, above. Thus, both the existence of object 600and its depth can be ascertained by measuring the change in theintensity of electric field 610 over a distance. The same process alsocan be used to detect objects by measuring changes in the intensity ofthe Earth's naturally occurring magnetic field. Similarly, by knowingthe distance above the surface traversed by the sensor probe and thedepth of the object will provide s relative dimension for the anomaly.

When a subsurface anomaly is present in the Earth, either or both of theelectric and magnetic field components can change based on the materialcharacteristics of the object. Consider, for example, non-magneticanomaly 700 having a relative permittivity ε_(r) and relativepermeability μ_(r) that differ from the permittivity ε_(e) andpermeability μ_(e) of the Earth in the vicinity of the anomaly, as shownin FIG. 7. When electric component 710 a of the Earth's electromagneticfield passes through non-magnetic object 700, the intensity of theelectric field will change in the vicinity of the anomaly, usually as anincrease in electric field strength. As discussed above, this change inelectric potential E of electromagnetic field 710 can be measured overan area or distance, as shown in FIG. 8 for frequencies f₁, f₂ and f₃,with frequency f₃ having the closest relationship to the depth ofnon-magnetic object 700 below ground-air barrier 720. However, becausethere is no perturbation of magnetic component 710 b of the Earth'selectromagnetic field, there will be no measurable change in theintensity of magnetic field M as shown in FIG. 8 due to the presence ofnon-magnetic object 700.

As a further example, the presence of a magnetic (paramagnetic toferromagnetic in classification) anomaly with relative permittivity morethan 1 and permeability more than 1 will cause variation with respect toboth the electrical and magnetic components of the Earth's magneticfield. FIG. 9 shows the presence of magnetic object 900 having arelative permittivity ε_(r) and relative permeability μ_(r) that differfrom the permittivity ε_(e) and permeability μ_(e) of the Earth in thevicinity of the anomaly within electromagnetic field 910. When electricfield component 910 a passes through magnetic object 900, the intensityof the electric field will change in the vicinity of the anomaly (again,usually, this is an increase in electric field strength). Additionally,the intensity of magnetic field component 910 b of electromagnetic field910 will change in the vicinity of the anomaly, with the change inmagnetic potential increasing or decreasing based on the orientation ofmagnetic object 900 vis-a-vis magnetic component 910 b ofelectromagnetic field 910. As shown in FIG. 10, both electric potentialE and magnetic potential M change in intensity in proximity to magneticobject 900 at frequencies f₁, f₂ and f₃. Because the electrical fieldradiates radially, it remains the most useful for determining the depthof magnetic object 900. The depth of magnetic object 900 belowground-air barrier 920 thus can be ascertained based on frequency f₃since it demonstrates the greatest change in intensity. Once the depthof the anomaly is discerned, the remaining electric and magneticcharacteristics can be used to determine the most likely materialcomposition (given a lookup table, or other suitable means forcomparison, is available). Thus, by measuring the changes in both theelectric and magnetic fields, a more accurate characterization can beperformed regarding the composition, size and material structure of theburied subsurface anomaly.

In one embodiment, a subsurface object may be detected through thedistortions in the electric field caused by differences in properties ofthe subsurface object and its surroundings. An example of this type ofdistortions resulting from an object with a strong dielectric constantin a static electric field is illustrated in FIG. 11. This figure showsthe electric field lines bending toward the region in space with higherdielectric property. In detecting subsurface objects, the static fieldlines may bend toward or away from the object depending on the materialof the object compared to the surroundings. The nature and severity ofthe distortion will be foremost characterized by the differences inmaterial properties between the subsurface object and its surroundings.Consequently, a passive detection method such as this one could be usedto locate anomalies on top of each other (with some earth between them)as well. The material properties that are very influential on a staticelectric field include conductivity, and dielectric permittivity. Theseproperties may, in turn, be affected by other properties, such asmaterial porosity and moisture content.

An apparatus with at least one, but possibly many, sensors can detectand measure distortions in one or more components the electromagneticfield as a function of position. The position of each measurement may bedetermined by using a network of known positions with a triangulationscheme, including global positioning system, or other appropriate means.The signals sensed at various locations over an area and the location atwhich a signal is sensed may be recorded and processed with anappropriately-configured computational device to estimate the locationand nature of the subsurface object.

For the field distortions to be measurable and to minimize the effectsof potential sources of electromagnetic and other forms of interference,the sensing probe should be in close proximity to the surface. This isbecause the density of the field's flux lines tend to re-equalize atlarge relative distances from a given object and thus the distortionsmay no longer be detectable if the probes are far away. In oneembodiment, non-geological scale objects having relative permittivityand permeability that differ from the permittivity and permeability ofthe Earth in the vicinity of the anomaly can be detected at near-surfacedepths at frequencies greater than 5 kHz, with the preferablefrequencies being a function of relative permittivity, which can rangefrom ε_(r)=1 to 100,000, relative permeability, which can range fromμ_(r)=1 to 1,000,000, and the resistivity p of the Earth, whichtypically is in the range of 10 to 1,000 Ohm-m and can vary up to 10times higher and lower in extreme situations.

In one embodiment, the signal is electromagnetic in nature and ismonitored over an area by several sensors. Monitoring the signal over asuitably sized area can be accomplished by mechanically scanning sensorsover the area, or by having a multitude of sensors distributed in somesuitable fashion over the region. FIG. 12 shows an exemplary sensorconfiguration 1200 comprising nine sensors 1210. Each sensor, or networkof sensors, may include a suitable arrangement of electronics andtransducers, such as electric field probes or magnetic pickup coils withappropriate amplification and conditioning electronics. Sensors also mayinclude permalloy sensors, including sheets of such sensors. Othersensors capable of sensing electrical or magnetic fields with sufficientsensitivity may be used, as would be understood to one of ordinary skillin the art. Such sensors could be in a handheld device (i.e. a scanneror metal detector sized apparatus), attached to a rotorcraft (such as RChelicopters or quadcopters), or another portable apparatus.

The sensor signals are preferably recorded and processed by acomputational device to extract the location of the subsurface object.The recording and processing is preferable done with a suitable computerand data acquisition system. The processing may involve computingdifferences in sensor responses as functions of sensor location, timethe signal is measured, and frequency of the signal. In one embodiment,the processing only concentrates on an electromagnetic signal of lowaudio and sub-audio frequencies, since attenuation of electromagneticsignal strength during propagation rises with increased frequency.Results of the computation then may be displayed in some fashion so thesubsurface object can be located. The display could be as complex as atwo- or three-dimensional map of the area scanned or as simple as anindicator light that activates on the sensing apparatus when over theobject.

In one embodiment, the sensor apparatus includes an array of probes,arranged spatially over an area in which distortions are to be measured.This arrangement may include a rectangular grid of probes, a hexagonaltiling, or some other regular or irregular arrangement. The probesthemselves can be embodied by a plate, field sensing dipoles, coils, orother structures composed of a suitable material. The probes should besuspended in close proximity to the ground, and be oriented to measureat least the vertical electric field component. This field componentshould be particularly strong due to the relatively high conductivity ofthe ground compared to the air above. (Electric field lines orientthemselves to impinge normally on “good” conductors.) If coils areemployed, horizontal magnetic field components may be of greaterinterest. Other field components may be measured, as well, andadditional information may be gained from measuring such fieldcomponents.

The suspension of the probes in close proximity to the ground should besuch as to not interfere with or obscure the field to be measured. Thesuspension framework thus should preferably be constructed of a materialthat has electrical properties as close to air as possible. For example,closed-cell extruded polystyrene foam and other hardened foamedmaterials have been found to possess reasonably suitable qualities.

As shown in FIG. 13, the signal collected from each of probes 1300 a-cmay be passed through its own frequency selective circuit 1310 a-c toreject signals not of interest prior to amplification of such signals byamplifiers 1320 a-c. In one embodiment, frequency selective circuit 1310a-c may include low noise band pass filters or selectively variablefilters to prevent overloading of amplifiers 1320 a-c by excessivelystrong undesired signals. After undesired signals are filtered orotherwise removed or otherwise addressed, each probe signal is amplifiedby amplifiers 1320 a-c. Each probe signal preferably is amplified by itsown high impedance instrumentation grade amplifier to recordable levels.The amplified signals are then transferred to a recording device 1330,where they can be stored, analyzed and interpreted. In one embodiment,the signal is digitized either before or after being transmitted orcommunicated to recording device 1330 and analyzed on a digitalcomputer.

In one embodiment, each probe, frequency selective circuit, andamplifier is a small self-contained unit that links to the recordingdevice, such as a computer, by a method that will not cause fielddistortions. A probe may include a small dipole antenna, an integratedcircuit chip that performs frequency band selection and amplification, aminiature battery, and a fiberoptic interface that carries the amplifiedsignal to the data recording device that has been prepared in such amanner as not to interfere with the signal to be measured. For example,the device could include of an electromagnetic interference (EMI)shielded laptop computer. Shielding may be accomplished by placing therecording device in a steel box at some distance from the sensing probesor in other ways that will be appreciated by those of ordinary skill inthe art.

In one embodiment, all probe signals are recorded simultaneously (inparallel) to remove the time varing randomness in the electromagneticfields used for sensing. The signals are then processed, preferably by acomputer with appropriate software, or some alternate mechanism. Theprocessing may include calculating one or more metrics to extract signaldifferences from probe-to-probe. Calculating a metric could, forexample, include decomposing the recorded signals into their frequencycomponents (spectral analysis) and then comparing probe to probevariations at various frequencies. If multiple field components aremeasured, techniques such as principal component analysis could also beused to determine the orientations of the largest field differences.

Comparing probes, spaced farther apart in the array, as opposed toprobes located adjacent, may allow relative depth probing. As discussedabove with respect to Equation 1, a relative correlation exists betweendepth and frequency, since electromagnetic field penetration into aconductive medium will decrease with increasing frequency (i.e., theskin effect).

In an embodiment, the sensor apparatus includes a stationary referencestatic electric field sensor, and one or more static field sensors thatcan mechanically scan over an area. The stationary field sensor's signalis used as a reference, to compare to other sensor readings. Themechanical scanning may be accomplished with a suitable x-y scanningmechanism. A field sensor may include a device such as an electric fieldmill as shown in FIG. 14, a field effect transistor with an appropriateprobe, or some other device that can properly sense the presence andorientation of a static electric field. In FIG. 8, the field millmeasures electric field 1400 using fixed electrodes 1410 that arealternately shielded and exposed to field 1400 by spinning rotor 1420,resulting in a modulation of induced electrical charge. Charge amplifier1430 can then convert the modulated charge into voltages proportional tothe strength of field 1400 that can then be measured by volt meter 1440.The electric field sensors should be suspended in close proximity to theground, and in one embodiment, are oriented to measure the verticalelectric field. The electric field on the surface of the Earth isoriented vertically due to the relatively high conductivity of theground with respect to the air directly above, as electric field linesorient themselves to impinge normally on “good” conductors. The fieldthus may be thought of as a charged parallel plate capacitor, where theEarth and the ionosphere comprise the plates of the capacitor, and theelectric field exists between the two. Mounting the sensors close abovethe ground should be such as to not interfere with the measurement ofthe static electric field.

Once the electric field component or other signals have been recordedas, for example, a function of position over the area as, for example, adigitized signal on a computer or microprocessor, they can be analyzedby suitable means to reveal distortions that indicate subsurfaceobjects, voids, or other anomalies. The results can then be displayed.In one embodiment, the results may be displayed as 2- or 3-dimensionalestimations of a subsurface object or its location, or as simple as anindicator light signaling the presence of a subsurface object.Recognition and classification techniques may be employed to furtherimprove the usefulness of the results for a given objective. Forexample, anomolies that meet certain criteria may be indicated in amanner that is different from other areas, such as highlighting an areawith characteristics consistent with a possible landmine or undergroundutility pipe, where as other subsurface objects such as rocks may beignored or shown in other colors or representations.

Various embodiments of the invention have been described above.Modifications, alterations, and/or combinations of the embodimentspresented will occur to others upon the reading and understanding ofthis specification. The claims as follows are intended to include allmodifications, alterations, and/or combinations insofar as they comewithin the scope of the claims or the equivalents thereof.

What is claimed is:
 1. A computer-implemented method for detecting asubsurface anomaly at a near-surface depth, comprising the steps of:positioning an electromagnetic sensor that is configured to measure acomponent of a planetary electromagnetic field such that theelectromagnetic sensor is suspended just above a ground-air barrier anddoes not contact a ground surface; selecting an electromagneticfrequency by calculating, with a computing device, a function ofproperties of the ground that include relative permittivity, relativepermeability, and resistivity; moving the electromagnetic sensor overthe surface of the ground; repeatedly measuring intensity of thecomponent of the planetary electromagnetic field at the frequency toobtain a set of measurements; and comparing, with the computing device,at least a first measurement in the set of measurements to at least asecond measurement in the set of measurements to identify a change inthe intensity of the component of the planetary electromagnetic fieldthat is indicative of a presence of a subsurface anomaly.
 2. The methodof claim 1, wherein the selected frequency is at least 5 kilohertz(kHz).
 3. The method of claim 2, wherein the subsurface anomaly ishuman-scale.
 4. A computer-implemented method for determining depth of asubsurface anomaly at a near-surface depth, comprising the steps of:positioning an electromagnetic sensor that is configured to measure acomponent of a planetary electromagnetic field such that theelectromagnetic sensor is suspended just above a ground-air barrier anddoes not contact a ground surface; selecting a first electromagneticfrequency by calculating, with a computing device, a function ofproperties of the ground that include relative permittivity, relativepermeability, and resistivity; selecting a second electromagneticfrequency by calculating, with a computing device, a function ofproperties of the ground that include relative permittivity, relativepermeability, and resistivity; moving the electromagnetic sensor overthe surface of the ground; repeatedly measuring intensity of thecomponent of the planetary electromagnetic field at each of the firstfrequency and the second frequency to obtain a set of measurements;determining, with the computing device, which of the firstelectromagnetic frequency and the second electromagnetic frequencyexhibits a greater change in intensity attributable to the subsurfaceanomaly; and calculating, with the computing device, a depth of thesubsurface anomaly; wherein calculating the depth is based at least inpart on measured intensity, at the electromagnetic frequency determinedto exhibit the greater change in intensity, of the component of theplanetary field.
 5. The method of claim 4, wherein the firstelectromagnetic frequency and the second electromagnetic frequency areeach at least 5 kHz.
 6. The method of claim 5, wherein the subsurfaceanomaly is human-scale.